1. Field of the Invention
The invention relates in general to a positioning apparatus disposed in the direct bottom type backlight assembly, and more particularly to the positioning apparatus for preventing deformation of a diffuser plate of the direct bottom type backlight assembly.
2. Description of the Related Art
With the advantages of handy size, light weight, low power consumption and no radiation contamination, the liquid crystal displays (“LCD”) whose display effect is much superior to that of a cathode ray tube display (CRT display) has attracted the public interest in recent years. A liquid crystal display comprises a display unit, a backlight unit, and a housing with a frame. The display unit comprises a display panel, a printed circuit board (PCB) capable of dealing with data signals, and a tape carrier package (TCP).
The backlight assembly is disposed under the display unit, for providing the light to the display unit uniformly. According to the light providing way, the backlight assemblies are divided into the edge light type and direct bottom type. Typically, a product of 20-inch size or less use linear light source (ex: Cold Cathode Fluorescent Lamp, CCFL, 2-8 of lamps in general) or spot light source (ex: Light-Emitting Diode, LED) disposed at the edge of the backlight assembly. Also, a light guiding plate and a plurality of diffusion sheets (optical sheets for enhancing the light) are used for increasing uniformity of light and lighting efficiency. Since LCD TV has a great demand on the brightness and the viewing angle, direct bottom type backlight assembly is widely applied in LCD TVs and the products of large size LCD monitors. Although the lamps required in the direct bottom type backlight assembly are more than that in the edge light type backlight assembly, light of the direct bottom type backlight assembly emits directly from the bottom of the assembly, and is uniformed by the diffuser plate (and diffusion sheets) without the light guiding plate. Thus, the direct bottom type backlight assembly comprises less backlight components.
FIG. 1 is a sectional view of a conventional direct bottom type backlight assembly. The direct bottom type backlight assembly at least comprises a housing 10, a reflective plate 12, the light sources 14, a diffuser plate 16 and a plurality of diffusion sheets 18. The linear light sources (for example, U-shaped lamp) 14 are arranged inside the housing 10, and the linear light is converted to planar light by the diffuser plate 16 and the diffusion sheets 18. The reflective plate 12 (for example, an aluminum plate) mounted on the bottom surface of the housing 10 is used to reflect light to the light output surface so as to improve the efficiency of the light. After the backlight assembly is completed, it is assembled with the display unit to establish a liquid crystal display (LCD).
The diffuser plate is commonly made of polymethyl methacrylate (PMMA), molybdenum disulfide (MS) or polycarbonate (PC). PMMA and MS possess better optical characteristics and anti-UV effect than PC; besides, PMMA and MS are much cheaper than PC. However, PC has a much better performance at the resistance of moisture shock. Under the environment of high temperature and high humidity, the diffuser plate made of PMMA or MS is moisture-retentive and bend easily. Therefore, the deformed diffuser plate interferes with display panel and causes the bad effect, so called as “pooling phenomena” on the display panel. Although the PC plate is moisture-resistant, the color of PC plate is easy to degrade (i.e. turn yellow), and the price of PC plate is much expensive than the PMMA plate and the MS plate.
FIG. 2 illustrates deformation of diffuser plate of FIG. 1 after moisture shock. The housing of FIG. 1 is eliminated herein. The bottom surface (i.e. the surface close to the light sources 14) of the diffuser plate 16 is directly heated by the light source 14, while the top surface (i.e. the surface adjacent to the display panel 21) of the diffuser plate 16 is not. Therefore, the evaporation rate of the bottom surface of the diffuser plate 16 is faster than that of the top surface of the diffuser plate. The difference between the evaporation rate of the bottom surface and the evaporation rate of the top surface causes the diffuser plate 16 to bend toward the display panel 21. Generally, a gap with a predetermined distance is arranged between the diffuser plate 16(/diffusion sheets 18) and the display panel 21, to provide a space for the deformed diffuser plate 16. After moisture shock, however, if the maximum deformation of the diffuser plate 16 is more than the distance space of the gap, the diffuser plate 16 presses against the display panel 21 and the pooling phenomena appears on the display image with the position corresponds to the touch point of the diffuser plate 16 to the display panel 21, this affects the display performance.
One of the common solutions is to attach the diffuser plate 16 to the housing by two-side adhesive tape, for maintaining the evenness of the diffuser plate 16. However, this conventional solution has the drawbacks such as difficult to assemble the diffuser plate 16 and the housing (for example, hard to attach the two-side adhesive tape at the tiny frame of the housing), and increase of the cost (i.e. labor cost for attaching the tape). Another one of the common solutions is to configure the grooves for positioning the diffuser plate 16; however, the diffuser plate 16 with a large suspending area absorbs moisture so as to cause considerable deformation especially at the center of the diffuser plate 16. If the diffuser plate 16 presses against the display panel due to the deformation, as shown in FIG. 2, the pooling phenomenon will appears on the display image. In the practical application, the gap (i.e. distance between the diffuser plate and the display panel) is determined according to the deformation degree of the diffuser plate. The larger the deformation degree, the wider the gap and the thicker the LCD.
Accordingly, it would be desirable to reduce the deformation degree of the diffusion plate during the thermal shock and moisture shock, so as to produce a product as thin as possible.